When salary or other income arrears are received in any particular year, one’s tax liability for that year increases. Simply because one’s total income for that year has increased. Most salaried individuals will be able to identify with such situations. But having to pay a higher tax on account of arrears is unfair to the taxpayer. Had he originally received the money in the year(s) that he was supposed to receive it, the additional tax would have been staggered over this time, instead of converging in one year as a lump sum payment.
Basically, the relief under Section 89(1) is arithmetical. It involves ascertaining the two amounts of tax, the first is the amount of tax applicable to the total income, including the extra amount in the year of receipt. The second is calculating the amount of tax by adding the arrears to the total income of the years to which they relate. The difference between the two amounts is the deduction allowed.